Effects of Chronic Static Stretching on Maximal Strength and Muscle Hypertrophy: A Systematic Review and Meta-Analysis with Meta-Regression

Background Increases in maximal strength and muscle volume represent central aims of training interventions. Recent research suggested that the chronic application of stretch may be effective in inducing hypertrophy. The present systematic review therefore aimed to syntheisize the evidence on changes of strength and muscle volume following chronic static stretching. Methods Three data bases were sceened to conduct a systematic review with meta-analysis. Studies using randomized, controlled trials with longitudinal (≥ 2 weeks) design, investigating strength and muscle volume following static stretching in humans, were included. Study quality was rated by two examiners using the PEDro scale. Results A total of 42 studies with 1318 cumulative participants were identified. Meta-analyses using robust variance estimation showed small stretch-mediated maximal strength increases (d = 0.30 p < 0.001) with stretching duration and intervention time as significant moderators. Including all studies, stretching induced small magnitude, but significant hypertrophy effects (d = 0.20). Longer stretching durations and intervention periods as well as higher training frequencies revealed small (d = 0.26–0.28), but significant effects (p < 0.001–0.005), while lower dosage did not reach the level of significance (p = 0.13–0.39). Conclusions While of minor effectiveness, chronic static stretching represents a possible alternative to resistance training when aiming to improve strength and increase muscle size. As a dose-response relationship may exist, higher stretch durations and frequencies as well as long program durations should be further elaborated. Supplementary Information The online version contains supplementary material available at 10.1186/s40798-024-00706-8.


Background
Stretch training is commonly used to achieve improvements in flexibility [1,2], with widespread applications in sports conditioning and orthopedic physical therapy [3,4].While it was widely accepted in the 1980s that static stretching should be included in warm-up routines [5][6][7], current evidence questions the implementation of (static) stretching during warm-up due to its detrimental impact on subsequent sports performance [8][9][10].
Despite adverse acute effects, static stretching may be beneficial for athletes if performed in the long-term [11,12].A recent systematic review with meta-analysis evaluating animal studies found chronic stretching of the anterior latissimus dorsi in chickens and quails (for up to 24 h per day, seven days per week) substantially increased muscle mass by up to 319% (d = 8.5) due to increases in muscle cross-sectional area (up to 142%; d = 7.9).Besides these structural changes, gains in maximal strength (up to 95%; d = 12.4) [13] were observed.Interestingly, investigations aiming to translate animals' muscle adaptions to humans were requested as early as in 1983: "Thirty minutes of stretching per day is certainly within normal physiological limits, and as a result may be applied to human muscle with hopes that similar adaptations would occur" [14].
Stretching effects on hypertrophy [15,16] and strength [17,18] in humans were previously reviewed pointing out only small strength increases (under dynamic conditions [17]) while muscle hypertrophy was exclusively evident using high intensity stretching [16].However, even though recent reviews were performed in 2023, they missed inclusion of new literature that -for the first time -applied static stretching with continuous stretching durations up to two hours [19][20][21][22][23][24][25][26], which might lead to an under-or overestimation of the current evidence.
Consequently, the aim of this systematic review with meta-analysis was to investigate changes in muscle size and maximum strength following chronic static stretching interventions in humans.We hypothesized that stretching programs, performed in the long-term, would lead to increases in both outcomes.Based on findings from animal research, we assumed that previous stretching volume was not sufficient.Therefore, we hypothesized longer stretching session durations and intervention periods, as well as high training frequencies would trigger improvements, while lower durations/frequencies would not elicit relevant changes.

Methods
A systematic review and meta-analysis using robust variance estimation was performed adhering to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines.The study was registered in the PROSPERO database (CRD42023411225).

Literature Search
Two independent investigators (KoW & LHL) conducted a systematic literature search using MEDLINE/PubMed, Web of Science and SPORTDiscus (March 2023) and updated in January 2024.The following inclusion criteria were applied: (1) randomized, controlled study design; (2) static stretching intervention with a duration of at least two weeks, performed in humans; (3) measurement of (a) maximal strength or related parameters such as active peak torque and/or (b) markers of muscle size (i.e., cross-sectional area, muscle thickness).Studies assessing acute effects, combining static stretch training with other (active) training protocols such as resistance training or neuromuscular facilitation, or including patients were excluded.The search terms (Online Supplemental Material) were created based on the requirements of each database.As an example, the terms for PubMed were as follows:

((stretch*) AND (performance OR strength OR 1RM OR force OR MVC OR (maxim* AND "voluntary contraction") OR hypertrophy OR "muscle crosssectional area" OR CSA OR "muscle thickness" OR "muscle mass" OR "muscle volume") NOT (acute OR postural OR pnf OR "proprioceptive neuromuscular facilitation" OR "stretch shortening")).
In addition to database searches, the reference lists of all included studies were screened for further eligible articles [27].

Methodological Study Quality and Risk of Bias
The assessment of study quality was performed by two independent investigators (KW1 & LHL) using the PEDro scale for randomized, controlled trials [28,29].
• Our systematic review is the first that included studies using comparable stretching durations of up to two hours in humans, which showed small magnitude maximal strength increases and muscle hypertrophy.
• Even though less effective, high volume stretching might provide a sufficient alternative to strength training when aiming to induce muscle hypertrophy and strength increases.It must be noted that comparatively high training effort is opposed by comparatively small adaptations, suggesting a preference for the more efficient strength training if applicable.Keywords Stretching, Exercise, Maximum strength, Hypertrophy, Long-lasting If consensus could not be reached, a third rater casting the decisive vote was consulted (MK).The PEDro scale (Table A in Supplemental Material) was used in previous reviews with meta-analysis on exercise and exercise therapy [30,31].
Risk of publication bias was examined using visual inspection of funnel plots [32], which were created using the method of Fernandez-Castilla et al. [33].Additionally, Egger's regression tests incorporating robust variance estimation for funnel plot asymmetry were applied [34].The certainty about the evidence was rated as very low, low, moderate or high using the criteria proposed by the GRADE working group [35].Generally, the quality of evidence of randomized trials is considered high and thereafter adjusted within the GRADE framework.In case of limitations in study design or execution, inconsistency of results, indirectness of evidence, imprecision or publication bias, one point is subtracted for each weakness.Conversely, large-magnitude effects or a dose-response gradient each lead to addition of one point to the quality of evidence rating.

Data Processing and Statistics
The means (M) and standard deviations (SD) from preand post-intervention tests were extracted for all parameters and study arms (stretching and inactive control).In case of missing data, the authors of the primary studies were contacted.Changes from pre to post were computed as M (posttest) -M (pretest) and standard deviations were pooled as To account for multiple within-study outcome dependency with unknown origin of covariances, meta-analytical calculation was performed using robust variance estimation [36].Standardized mean differences (SMD) and 95% confidence intervals (CI) for maximal strength capacity and muscle size changes (including both muscle thickness and muscle cross-sectional area) were pooled from fitting parameters from all included studies.We used R (R Foundation for Statistical Computing, Vienna, Austria) with the robumeta, version 2.0 [36] and metapackages.Obtained effect sizes (ES) were interpreted as 0 ≤ d < 0.2 trivial, 0.2 ≤ d < 0.5 small, 0.5 ≤ d < 0.8 moderate, or d ≥ 0.8 large [37], while τ² was used to explore study outcome heterogeneity, with classifications equal to effect sizes.Meta-regression was performed using the robumeta package for dependent study outcomes, as described by Fisher & Tipton [36].Furthermore, to quantify the influence of quantifable outcome moderators (stretching duration, intervention period and training frequency) when aiming to enhance maximal strength and muscle size, sub-analyses were performed for three variables: intervention duration, session duration and exercise frequency.For moderating variables (duration, intervention period and training frequency), we used the median-split for cut-off determination (intervention duration: small: <6 weeks vs. high: ≥ 6 weeks, frequency: low: <5 sessions vs. high: ≥5 sessions, stretching duration: short: <15 min vs. long: ≥15 min.To test for significant differences in mean effect size of sub-groups, the Welsh test was performed due to violation of normal distribution.If several study effects were presented mean effects for each study were calculated to account for within-study dependency in effect size comparsions.

Search Results
Figure 1 displays the flow of the literature search.

Methodological Quality, Risk of Bias and Quality of Evidence
Per average, the methodological quality of the included studies was rated as fair [72] (mean 4.17 ± 1.4 out of 10 points; range 2 to 8 points; see Table A in Supplemental Material).For both outcomes (muscle volume and maximal strength), the quality of evidence was downgraded by 2 levels (high to low) due to high risk of bias (limitations in study quality: fair PEDro score and heterogeneity in study designs).In case of the sub-analyses for session and intervention duration (outcomes of maximal strength), the quality of evidence was upgraded by one level due to moderate to strong associations (low to moderate effect sizes, mostly on same side effect).

Quantitative Synthesis
Table 1 provides the study characteristics of included articles, while Table 2 summarizes the quantitative analysis results for overall and different subgroups.

Hypertrophy
For hypertrophy, a trivial positive effect of stretching was found (d = 0.20, p = 0.003, 95% CI 0.08 to 0.32, τ²=0.0,19   4).The certainty about the evidence is low.While the meta regression (p = 0.23-0.88)revealed no significant influence of any included moderator, long-duration stretching (≥ 15 min) had a small effect size (d = 0.28, p = 0.005, 95% CI 0.12 to 0.44, τ²=0.0, 7 studies, 17 ES) without a significant difference compared to shorter durations (p = 0.29) that, in turn, failed reaching a significant effect (d = 0.13, p = 0.14, 95%CI -0.05 to 0.30, τ²=0.0, 12 studies with 28 ES).Similarly, studies that performed stretching for more than 6 weeks revealed d = 0.26, p < 0.001 extracted from 16 studies with 35 ES, while shorter training periods failed to reach the level of significance (d= -0.05, p = 0.13 from 3 studies and 10 ES) with higher effects for longer periods (p = 0.006).If stretching was performed more than 5 times per week, there were significant small magnitude increases in muscle size (d = 0.27, p = 0.002, from 11 studies with 28 ES), opposed by no significant effect for lower training frequencies (d = 0.09, p = 0.39), without a significantly higher mean effect size for higher frequencies (p = 0.31).The certainty about the evidence is low for all effects.

Publication Bias
Visual inspection of funnel plots (Fig. 5) revealed no indication of a publication bias for maximal strength as well as for muscle volume.Consistently, for both outcomes,

Discussion
In accordance with previous research, the present systematic review found chronic static stretching to increase (a) maximum strength [11,12,17,18], and (b) muscle size [16].With stretching duration and a tendency for intervention time as moderating training parameters for maximal strength, our results indicate longer stretching durations to be of superior effectiveness.While overall stretch-induced hypertrophy showed small effects (d = 0.2), these effects seem attributable to stretching durations of ≥ 15 min, intervention periods of > 6 weeks and training frequencies of ≥ 5 times as lower dosage did not reach the level of significance in subgroup calculations (p = 0.14-0.39).The possible necessity of high stretching volumes with regard to improvements in strength and muscle volume is in line with results from animal studies [73,74].
As pointed out, early evidence had mostly suggested that stretching does not modify morphological and functional muscle parameters in humans [11,12,15].However, this assumption was based on a lack of studies using high to very high stretch durations.Even the most recent review of Arntz et al. [18] did not include long duration studies [19-21, 25, 26, 75, 76], while Panidi et al. [16] included only one long-duration study [26].Since animal research indicated a potential dose-reponse relationship [14,77], a meta-regression was performed that confirmed stretching duration to significantly moderate strength adaptations.While in contrast, the regression did not reveal such a relationship for muscle hypertrophy, significant muscle size enhancements were only obtained in higher dosage in subgroup analyses (≥ 15 min stretching, ≥6 weeks intervention period, ≥5x stretching per week).Compared to animals with reported muscle mass increases of up to 300% [78], human hypertrophy effects must be considered small.These differences could be attributed to diverse factors.Compared to animals, human muscle protein synthesis is slower [79][80][81].This may be one explanation for a lack of hypertrophy in response to 30 min of stretch reported by Yahata [65].Nevertheless, by using stretching durations of accumulated 15 min per session, Wohlann et al. [20] obtained significant muscle hypertrophy.There were differences in the intervened muscle groups, Wohlann used 4x weekly pectoralis stretching, while calf muscle stretching performed by Yahata and colleagues [65] was applied only twice per week.The potential role of training frequency is supported by consistent hypertrophy effects in all Warneke et al. studies [23,24,26], who used daily stretching.The results of the meta-analysis partly confirm this assumption, although meta regression did not reach the level of significance for both, maximal strength and hypertrophy.However, subgroup analysis for hypertrophy showed only more frequent training application to produce significant effects, while no significant influence of frequency was observed for strength increases.
Several mechanisms could explain the stretch-induced increases in muscle size or strength.First and foremost, it may be speculated that time under tension is not only paramount for gains in muscle volume following resistance training [82] but also following stretching [83], which would be in agreement with our results, showing the stretching duration to be important for strength (meta regression: p = 0.038), but also for hypertrophy, as only with ≥ 15 min muscle size did increases occur.Accordingly, the literature shows high mechanical tension imposed on the sarcomere could trigger protein synthesis [84,85].In quails and chickens, progressive stretching induced fast hypertrophy alongside serial sarcomereogenesis during the first days of the intervention [78].However, when the stretching stimulus remained unmodified during such a program, initial increases in muscle cross-sectional area started to disappear [86].Ashmore [87] suggested that the mechanical tension caused by stretching would lead to high stresses and compensatory adaptations in the sarcomere.It has, furthermore, been hypothesized that an increased total amount of sarcomeres reduces tension and with this stress on the individual sarcomere [86].Thus, to increase training intensity and to ensure continuously strong tensioning of the sarcomere, the stretching stimulus needs to be re-adjusted.Indeed, Antonio & Gonyea [78] achieved the highest gains in muscle mass and hypertrophy by increasing the stretch intensity, starting with 10% of the body weight up to 35% after 5 weeks of chronic stretch in quails.
Another theory postulates that chronic stretch creates hypoxic conditions which are similar to those during blood flow restriction.Reducing arterial perfusion has been demonstrated to increase lactate levels, growth hormone concentrations, and inflammatory cytokines such as interleukin-6 [88,89].Such metabolic milieu may represent a potent stimulus for mTOR signaling [90][91][92].Interestingly, Jessee et al. [93] showed that blood flow restriction induces hypertrophy, however, it seems of minor relevance for maximum strength increases.Hotta et al. [94] observed acute decreases of blood flow during 30 min of stretching in animals.Studies measuring the metabolic muscle response to stretching would thus be warranted in order to further delineate the potential relevance of the abovementioned factors.
In sum, irrespective of initial processes, muscle hypertrophy requires an increase in muscle protein synthesis.Suzuki & Takeda [95] and Kremer [96] described the activation of stretch-activated channels and thus, the stimulation of the mTOR/p70S6K/PI3K pathway [97][98][99].The literature emphasizes the importance of mechanical tension (e.g., through stretching) to trigger anabolic signaling pathways, with the stimulation of protein synthesis [100][101][102][103] as an underlying mechanism of hypertrophy (and maximal strength) [104][105][106].Van der Pjil et al. [107,108] indicated the relevance of titin unfolding in hypertrophy (in parallel and longitudinal), supporting the hypothesis of high intensities [109].Conversely, Fowles et al. [110] were not able to show acute increases in protein synthesis after 33-minutes of stretching in humans, although significant increases in protein synthesis rates had been reported in animals [100,102,103,111].The stronger response in animals could hence be explained by a higher protein synthesis rate [80,81].With regard to the increases in maximum strength, it may be expected that the increases in muscle volume would drive the strength gains.This would require hypertrophy to precede enhanced strength.However, no study has investigated the temporal association of both factors.In addition, effect sizes were trivial to small for muscle volume but moderate for strength.Another theory may attribute the improvements to neural adaptations [112,113].The studies by Warneke et al. [19,26] and Nelson et al. [60], on the one hand, provide support for this assumption as they detected strength increases in the non-stretched contralateral leg.However, on the other hand, Holly et al. [114] and Barnett et al. [115] showed no significant increase in EMG activity during stretching in animals.Furthermore, Sola et al. [116] found stretchmediated hypertrophy in denervated muscles, indicating a minor role of neural aspects.Therefore, to clarify the role of neural aspects in stretch-mediated adaptations, further research seems necessary.
Even though muscle hypertrophy only occurs using higher dosage stretching, our work has significant clinical implications.In general, stretching may represent an alternative to conventional resistance training interventions inducing muscle size-and strength increases.Nevertheless, several aspects must be considered.While Currier et al. [117] showed moderate to large magnitude maximal strength and muscle size increases of ES = 0.51 and ES = 1.60, respectively, when using resistance training, the present study's small magnitude effect sizes of ES = 0.28 and ES = 0.45, respectively, showed that even long stretching durations were less effective.Assuming about one hour of stretching on one isolated muscle to achieve meaningful muscle hypertrophy [83] seems, on the one hand, of limited practical relevance [85].On the other hand, passively induced mechanical tension via stretch training could be included into daily life, with for example using splints/ortheses during sitting in the office or while watching television [118].A further benefit might be the potential applicability for people lacking motivation or ability to perform resistance training (e.g., patients with unstable cardiovascular diseases), if heavy resistance training is contraindicated, or after muscle, ligament or bone injuries leading to prolonged times of immobilization.Thus, (probably only) for conditioned populations, stretching could provide a sufficient alternative, especially since no training supervision is necessary to ensure safe exercise execution.Although stretching could be a valuable training intervention, it should only temporarily substitute or, even better, supplement classical training regimes.This is of importance because although stretching has been shown to be beneficial for cardiovascular health [119], it may not add as efficiently to the recommended levels of physical activity (e.g. by the World Health Organization, 150 min of moderate or 75 min of vigorous activity per week) as other activities such as walking, running, team sports, or resistance training.
Several aspects call for further research.Even though significant stretch-induced muscle hypertrophy in response to stretching durations of ≥ 15 min was identified, this was based on only 7 studies with a range of 3 × 5 min to one hour of stretching, with the highest effects originating from one research group [19-21, 23-26, 76].Thus, further studies are requested to confirm or disconfirm the results.Furthermore, all long-lasting stretch interventions (more than one hour) were performed with high stretching frequency and intervention periods (≥ 6 weeks), increases in maximal strength and muscle volume cannot be clearly ascribed to one of these parameters.Further studies should hence examine longlasting stretch interventions of < 6 weeks and/or ≤ 5 sessions per week.Moreover, the role of stretch intensity merits further investigation.Reporting stretch intensity using individual pain perception seems of questionable validity [120].However, it is well known from strength training that training intensity seems to be of crucial importance for adaptations, especially with regard to maximum strength increases [121].Considering the importance of titin unfolding, which is assumed to occur exclusively in maximally stretched sarcomeres, reaching high degrees of stretch could be hypothesized to be of paramount importance [109,122].
Despite some plausible theories [83], the underlying mechanisms remain speculative.While many physiological parameters were assessed in animals, no studies examined signaling pathways and possible alterations of protein synthesis in humans.Furthermore, research has almost exclusively focused on skeletal muscle.Interestingly, it has been shown that the connective tissue can exert significant force transmission effects [123].Therefore, it may be prudent for future trials to consider multiple tissues.
Some increases in the examined parameters were surprisingly high in studies included in our review.Nelson et al. [60] reported an improvement in maximal strength of 29% (d = 1.48) in the stretched leg and a gain of about 11% (d = 0.46) in the contralateral control leg following 4 × 30 s stretching three times per week for ten weeks.Mizuno [55] found increases of 24% using static stretching three times per week for eight weeks, while Panidi et al. [69] detected hypertrophy effects of up to 23%.When these short duration stretching results are compared to those from strength training [124], the listed stretch-induced adaptations seem unreasonably high, even though participants are partially classified untrained to recreationally active.Against this background, it will be of interest to further identify moderator variables determining strong and weak stretch responders.
Lastly, testing for significant differences of mean effects to provide a valuable statement of subgroup differences was performed using the Welsh test.This testing procedure must be considered a supplementation of the main statistics and must be interpreted with caution, as no specific pooling for dependent outcomes was possible.If one study provided multiple outcomes, effect size means were calculated, meaning each study corresponded to one outcome, which reduced this limitation.

Conclusions
The present systematic review provides low-to moderate-certainty evidence that chronic static stretching increases maximum strength and muscle size.While the overall effects are small if existent, comparatively high effort seems necessary with longer stretching-and intervention periods (≥ 15 min, ≥ 6 weeks) and greater frequencies (≥ 5x/week) seem particularly effective.The exact physiological mechanisms causing potential effects remain a matter of debate.Nevertheless, even though less effective compared to resistance training, high volume stretching might provide a valuable alternative under special circumstances, e.g., if traditional resistance training is contraindicated.

Fig. 3 Fig. 4
Fig. 3 Illustrates the meta-analytical results of short stretching durations.Legend: HI = high intensity group, LI = low intensity group, 1RM = one repetition maximum

Fig. 5
Fig. 5 Shows funnel plots for visual publication bias inspection, with (a) for maximal strength studies and (b) for hypertrophy studies.Plot size illustrates the number of outcomes in the respective study that were pooled and weighted in the meta-analytical calculation